CNG VEHICLE ACCIDENT IN FOCUS: ISSUES, CAUSES AND SAFETY RECOMMENDATIONS – an article by Mr. Asim Riaz, Research Engineer/Visiting Lecturer MS Energy Engineering, International Islamic University, Islamabad, Pakistan
Over recent weeks 44 people have died in accidents involving public sector transport vehicles converted to compressed natural gas (CNG) in Pakistan, the country with the world’s largest population of natural gas vehicles.
Before discussing the technical aspects of the problem in hand, it is important to mention two things right at the outset which relate to inadequate awareness and incorrect perceptions about the CNG cylinders. It was surprising that electronic and print media persisted in erroneous reporting that the accidents occurred due to bursting/explosion of the CNG cylinders. Add to that, some of the Government departments were also relating these accidents to non-compliance of Hydrostatic testing of CNG cylinders. Both are absolutely wrong impressions. This incorrect reporting has continued to date which has developed serious apprehensions about CNG cylinders in general public.
In the context of the foregoing, the following points are elaborated for clear and correct understanding of the problem:
a) Contrary to the media reporting no approved CNG cylinder has ever exploded in any accident of the CNG converted public service vehicles in the history of Pakistan CNG Industry. Approved CNG cylinders having manufacturing standard NZS 5454 should never burst or explode and so was the case in recent accidents that claimed 44 lives. The few CNG cylinder burst accidents that happened in the past were all in private vehicles, not in the public service vehicles.
b) Non-compliance of Hydrostatic testing of CNG cylinders after the due date was not the cause of any cylinder accident that ever happened in Pakistan. There is no correlation between CNG accidents and non-compliance of Hydrostatic testing requirement of CNG cylinders; hence relating these accidents with such testing of CNG cylinders is totally incorrect.
c) CNG Safety Rules, 1992, do not cover all the required safety measures to prevent CNG vehicle accidents similar to that of last month which claimed precious lives. These rules require updating.
On the spot examination of vehicles and their damaged CNG kits after the accidents [together with conversion facility inspections, practical experiments, reading of technical manuals and international standards] have helped in establishing some causes due to which the vehicles caught fire and identifying possible solution to prevent the causes of such accidents.
Causes of Accidents in the CNG-Converted Vehicles/NGVs:
The fire accidents in the CNG converted vehicles were caused by the leakage of gas after collisions of vehicles and in some cases even without any collision. The leakage of gas and resultant catching of fire is attributable to following factors:
1. Leakage of gas from high pressure piping due to:
a. Breaking of high pressure piping;
b. Dislodging of high pressure fitting;
c. Damaging of high pressure pipe due to impact, mechanical wear and tear and constant vibration; all these can occur without a collision of the vehicle.
d. Improper installation of high pressure pipe and fitting. It can result in damage and dislodging of high pressure pipe and fitting.
2. Detaching of CNG cylinder.
3. Sparking of air-gas mixture created in the passenger cabin can be due to cigarettes, lighter, match box, electrical instruments etc carried by the passengers.
4. After the collision the unwanted ignition could be caused by:
a. Short circuiting of battery and electrical wiring;
b. Contact of fuel with hot surfaces i.e. engine, exhaust manifold, radiator etc;
c. Spark created when kinetic energy of motion is converted into heat energy.
5. Bursting of unapproved cylinders, welded cylinders and cylinders that are not manufactured for CNG use.
6. Bursting of unapproved valve.
7. Leakage of gas from the cylinder valve.
8. Leakage of gas from the filling valve
9. Bursting/leakage of gas from the CNG Pressure Regulator due to malfunctioning of the first stage valve (Note: Bursting only if 1st stage safety is replaced with dead plug)
Recommendations to Prevent the Accidents in CNG Vehicles
Almost all CNG incidents in Pakistan to date could have been prevented if suitable precautions had been in place and the systems were fitted and inspected by qualified personnel. All CNG vehicles should be inspected by Hydrocarbon Development Institute of Pakistan (HDIP) on completion of the original conversion and all CNG vehicles (including OEM fitted) must be inspected at least once every five years by HDIP or any time the vehicle is involved in an accident or fire
The writer has more than 8 years of experience in designing, evaluation, inspections and accident reporting of CNG station and CNG vehicle system/equipment. He was deputed to make technical reports of some accidents to ascertain reasons for these accidents and to recommend safety measures for the future.
Opinions expressed in this item are those of the author. Publication by NGV Global does not imply endorsement or agreement.